Beginning with the advent of electrical generation and consumption, electrical conductors, wires, termed transmission lines, have been used to carry electrical power from the place of generation or storage to the device, the "load", which uses or consumes that power. The transmission line structure varies in its form, in part depending upon the kind and type of electricity that is being carried and the particular environment through which it passes. At high alternating current frequencies, RF, and above, such as in the range of 2 thru 18 GHz, the transmission lines, more particularly the "RF" transmission lines, used for this purpose generally include waveguide, coaxial, parallel wire (micro strap) and micro strip transmission lines all of which contain two conductors, one of which is regarded as the electrical "common" or "ground". The general structure and characteristics of those RF transmission lines are known to those skilled in this art.
In many cases the RF transmission line carries RF electricity over long distances, and in other cases not. As known to those skilled in the art, the transmission line is also used to connect RF energy between individual component elements within a particular electronic device, such as components in an electronic amplifier, that are located perhaps no more than an inch or two apart. In these applications, the connecting transmission line may be referred to as an RF connecting strip. The present invention addresses a new structure for an RF connecting strip.
The general purpose of the electrical roadway, s to speak, is to carry the RF energy from one location to another reliably and with minimal loss and maximum transfer of the electrical energy.
To the designer, one important property of a transmission line is its characteristic impedance, Zo. That impedance is dependent upon the physical structure and dimensions of the conductors, the dielectric constant of the insulating materials separating the conductors and the conductivity of the wire itself. Maximum power is transferred between the transmission line and the electrical device which is to receive the electricity, the "load", when the lines characteristic impedance is equal to the impedance of the load. Power transfer is important for several reasons. If power is lost either by dissipation or by reflection, due to impedance mismatch with the load, the power may need to be made up in other ways, such as by the addition of power amplifiers. This is a "fix" that is expensive, adds additional complexity, size, and weight to the equipment; all important factors in the useful electronic arts. In addition, the energy reflected due to a mismatched interface interacts with the forward energy forming a frequency dependent amplitude characteristic. This frequency dependent amplitude characteristic or amplitude ripple can be very difficult to eliminate and can seriously degrade the performance of microwave systems.
Temperature plays a role in the design and use of transmission lines and electronic amplifiers and the like. As temperature increases, it usually causes materials to expand or lengthen and different materials do so to different degrees. Conversely when the temperature drops, the materials contract. This causes mechanical stress cycling of the electrical interface structure for which the designer must account. Moreover repeated temperature cycling of the equipment results in repeated expansion and contraction which will result in material fatigue and might cause electrical connections between parts of the electrical transmission circuit to break and open, rendering the circuit inoperative. Since repair is always time consuming and expensive, if the connections withstand that cycling, the advantages are obvious. Stresses of the type described above in connection with temperature can also result from exposure to mechanical shock and vibration environments. The subject invention provides essential stress relief to preclude the former from impairing the integrity of the electrical connections.
One may select conductors from an innumerable list of metals and alloys, the dielectric from a similar host, the geometry, the length and so on; and such variables present infinite possibilities to the designer. The present discovery, if viewed as an extraction from the realm of infinite possibilities of lines with RF power connecting possibilities, is of a singular structure that provides reliable coupling in the hostile environment described with negligible effect upon the electrical transmission characteristics. The novel transmission line maintains a relatively constant RF characteristic as the environment and the transmission line and associated system undergoes physical changes due to thermal and mechanical effects upon the line and upon the associated equipment with which it is connected, and is both simple and inexpensive. The present invention enhances the reliability of the electronic circuits of which they form a part, resulting in lesser initial construction cost and/or eventual maintenance cost, and maintains those circuits true to the optimum design performance parameters.
One example of a simple RF interconnection for discrete electronic components is presented in U.S. Pat. No. 4,600,907 granted July 15, 1986 to Grellman et al. In Grellman a connection for two semiconductor devices uses a co-planar microstrap waveguide that includes a plurality of thin straps of conductive metal embedded in a polyimide substrate, an electrical insulator, and is dimensioned to exhibit the electrical transmission characteristic of a co-planar waveguide. At one end, the strips are bonded to one component and at the other to the leads on a circuit board to which the component is attached. The strip forms an incline extending from one level to the circuit board at a lower vertical level. The co-planar line in Grellman is useful in applications in which a ground plane or ground connection is not accessible. Not only is the ground plane of the devices in those applications unavailable, but the substrate is usually fabricated of a hard material with thermal expansion characteristics similar to that of the die thereby minimizing thermal stresses and allowing direct inflexible die bond attachments of the microwave semiconductor strip.
In the present invention, the connection strip is designed for a low loss, low voltage standing wave ratio interconnection between soft microstrip substrates and rigidly mounted microwave devices thereby requiring mechanical flexibility characteristics. The particular arrangement in Grellman interconnects a chip to a hard substrate and thus requires a minimum of mechanical stress relief. Grellman thus does not address the need for an effective microstrip interconnection that addresses the problem that has plagued the microwave industry: The need for a connection strip design that is simple and inexpensive to fabricate and that, simultaneously, solves the electrical and mechanical performance problems in an environment where wide changes in temperature occur and mechanical shock and vibration are present. Not only does Grellman present a different structure, that structure is designed for application in a different environment than that intended for the present invention. Other representative connecting strip transmission line structures used to couple microwave energy between circuit elements appear in the patent literature. This includes U.S. Pat. No. 4,288,761 granted Sept. 8, 1981 to Hopfer, U.S. Pat. No. 4,280,112 granted July 21, 1981 to Eisenhart, U.S. Pat. No. 4,260,963 granted Apr. 7, 1981 to Drapac, U.S. Pat. No. 3,969,689 granted July 13, 1976 to Corrons, U.S. Pat. No. 4,001,834 granted Jan. 4, 1977 to Smith and U.S. Pat. No. 3,806,767 granted Apr. 23, 1974 to Lehrfeld.
A principal object of the invention is to provide a new physically minute microstrip type RF connecting strip and to provide a combination of discrete components and an RF connecting strip connected between those components. A further object of the invention is to provide an RF connecting strip that is of simple geometry, which may be easily fabricated and which possesses RF transmission characteristics at a given frequency that do not change in any significant degree when the ends are pulled to change the strip's geometry or compressed to do so. A still further object is to provide an RF connecting strip that is capable of attachment to discrete components and, as attached, is capable of withstanding repeated stress and compression during repeated temperature cycling and exposure to mechanical shock and vibration without separating from the components to which the strip is attached.